It shouldn't happen often.
Also, Sindragosa and Halion are two very different encounters for tanks. I wouldn't lump them together.
Halion does hit very hard with his melees, I know. The damage for Halion is physical, fire and shadow. The latter two depending on the realm you are in. Of course you already know this, but bear with me.
Sindragosa is different in that the magical damage during that encounter are frost and arcane. At the start of WotLK, players were presented with the frost resistance crafted set(s) specifically for Sapphiron and Kel'Thuzad (a design decision Blizzard didn't go forward with in later tiers). I have personally tanked Sindragosa on both a DK and a Paladin, multiple times over, for a number of years. On all difficulties, as well. Almost always, I use my frost resistance set. This set on my Paladin puts me at 37k armor, 21 expertise. On my DK, I am at 33.9k armor and 25 expertise. Self-buffs.
I've never died to a parry-hasted attack from Sindragosa. Ever. That's not to say she hasn't parry-hasted me, but just that damage is never what would kill me on that fight. There are other things in that fight that are far more threatening. Hence the frost resistance set.
Halion is very different because there isn't an effective usable shadow or fire resistance set. And of course his melee attacks hit harder than Sindragosa. I'd be worried if they didn't, considering he is a full half-tier above Sindragosa. But it's also a fight that both the tanks and the healers have to change their approach in order to handle it well. Flame Breath hurts, and there isn't a resistance set for it. But if none of your raiders are standing in fire, then there's not going to be any damage going out aside from the occasional debuff and the tank damage. That changes in the shadow realm of the fight, but the bulk tank damage is still a concern and is usually the focus of the healers.
I have died to parry-hasted attacks on Halion, but that has only ever been in combination with a breath that I didn't have a CD for.
TLDR; On those fights, parry-hasted attacks shouldn't be too common, and they shouldn't be life-threatening. It's the magic damage you should be more worried about.